1. Field of the Invention
The present invention relates to an Al-killed steel sheet having excellent workability and a production method thereof. More particularly the present invention relates to an Al-killed cold rolled steel sheet containing boron and a method for producing a cold rolled steel sheet having excellent cold workabilities by subjecting a hot rolled steel sheet to cold rolling and continuous annealing.
2. Description of the Prior Art
Conventionally, the Al-killed steel sheet has been widely used as a deep-drawing cold rolled steel sheet for the reason that AlN which precipitates during the annealing forms a recrystallization texture which is favorable for deep-drawing and the aluminum fixes the nitrogen to make the steel non-ageing. However, in order to obtain the excellent deep-drawability, it is necessary to slowly heat the steel at a temperature between 400.degree. and 600.degree. C at a heating rate of about 10.degree. C/hour or to maintain the steel at this temperature. For this purpose, a box annealing has been conventionally practiced, which requires a considerable amount of time, e.g. as long as several days.
As a method for shortening the annealing time, a continuous annealing may be used instead of the box annealing. However, when Al-killed steel after cold rolling is subjected to a continuous annealing, the r value (Rank Ford value), which represents deep-drawability, is lower than that obtained by the box annealing because the heating rate is remarkably high in the continuous annealing. Even so, the recrystallization grains are remarkably smaller than those of a rimmed or capped steel so that an Al-killed cold steel sheet as continuously annealed is too hard to be used for deep-drawing.
For overcoming this defect, it has been known to increase the coiling temperature above 680.degree. C (hereinafter called a high coiling temperature). Even in this case, the grains after the continuous annealing are still small, sufficient softness is not obtained, and the r value is low. Also, the acid pickling efficiency lowers due to the high coiling temperature and the harmful effect of the lump coalescent cementite remains even after the cold rolling and continuous annealing.
Thus, regarding the annealing after the cold rolling, when the annealing is conducted below the A.sub.1 point, the lump cementite remains as it is. On the other hand, when the annealing is conducted above the A.sub.1 point, a lump pearlite appears so that the cold steel sheet which is coiled at high temperature is not desirable for materials used for applications where local deformation ability is required, such as, local stretching.